1. Field of the Invention
This invention relates to light pulse detecting devices for detecting quantity of pulsatingly changing light and, more particularly, to light pulse detecting devices most suited to light encoders.
2. Description of the Prior Art
In the past, this kind of device was constructed as shown in FIG. 1, where a reference voltage source Vref is connected to the base electrode of an NPN transistor 1 of which the emitter is grounded through a resistor 2 and the collector is connected to the input of a current mirror comprised of PNP transistors 3, 4 and 5, so that a current equal to the collector current of the NPN transistor 1, flows out from the collector of the transistor 4 which is the output of the current mirror. The collector of the transistor 4 is connected to the collector of a photo-transistor 6 with their junction el connected to the positive input terminal of a comparator 7. The emitter of the photo-transistor 6 is grounded. The negative input terminal of the comparator 7 is connected to the reference voltage source Vref.
The operation:
The voltage across the base-emitter path of the transistor 1 is assumed to be VBE. Then, a voltage of Vref - VBE appears across both ends of the resistor 2. So, a current flowing from the emitter of the transistor 1 has a value of (Vref-VBE)/R2, where R2 is the resistance value of the resistor 2. Also, an almost equal current flows to the collector of the transistor 1. By the current mirror of transistors 3, 4 and 5, the collector of the photo-transistor 6 is supplied with current of intensity almost equal to (Vref-VBE) / R2.
At first, the photo-transistor 6 is exposed to light so intense that the collector current of the photo-transistor 6 exceeds (Vref-VBE) / R2. As the potential at the collector of the photo-transistor 6 then reaches the saturation voltage of the photo-transistor 6, falling below the reference voltage Vref, the comparator 7 produces an output in the form of the low voltage. Next, as the light quantity on the photo-transistor 6 is not so large, when the intensity of the collector current of the photo-transistor 6 decreases from (Vref-VBE) / R2, the potential at the collector of the photo-transistor 6 changes to (Vcc - VCEsat4 (saturation voltage of transistor 4)), being higher than the reference voltage Vref, and the comparator 7 changes its output voltage to a high level.
For pulsating variations of the light on the photo-transistor 6, the potentials at the various points in the circuit of FIG. 1 take the waveforms shown in FIG. 2.
As the light varies in the form of pulses hv, the potential el at the collector of the photo-transistor 6 varies from its saturation voltage to VCC (the battery voltage)-VCEsat4. Because of a parasitic capacitance of the photo-transistor 6 and transistor 4, as shown on line el in FIG. 2, its rising and falling are delayed. Since, in this case, the range of variation of the potential el is so large, from the saturation voltage of the photo-transistor 6 to VCC-VCEsat4, the amount of charge necessary to charge-to-discharge the parasitic capacitance also becomes large. Thus, the delay of the rising and falling of the potential el takes a very long time. For this reason, the time gap tl, from the moment at which the rising edge of the light pulse has occurred to the moment at which the comparator 7 changes its output, tends to increase objectionably. Another drawback which was very serious was that as the width of the light pulse and the light quantity incident on the photo-transistor 6 decrease, the potential el becomes unable to reach the reference voltage Vref of the comparator 7 during the time when the photo-transistor 6 is exposed to the light, making it impossible to detect the incidence of the light pulse.
In application of such a light pulse detecting device to the light encoder, therefore, the possibility of failing to detect the light pulses was very high. So, an attempt has been made to improve the response characteristic of the light pulse detecting device by increasing the intensity of current flowing to the light-emitting element or LED of the light encoder with increase in the intensity of emitted light so that the intensity of light on the photo-transistor 6 increases.
However, the use of this method leads to consume a very large amount of electrical energy by the light-emitting element when a train of light pulses are produced. Such a light pulse detecting device was, therefore, not suited to be used in instruments of small size, such as cameras, whose electrical power source is a dry battery of small capacity, as constituting part of a light encoder, for example, the conversion of the moved amount of the diaphragm control ring or the axial movement of the focusing or zoom lens unit to an electrical signal by using a photo-interrupter.